Time Machine backups are unfussy: unlike many proprietary backup software packages, you can browse a Time Machine backup either through its interface or via the drive in the Finder. However, it appears as if every backup is a complete backup.

This concerned Macworld reader Peter, who just purchased a new iMac and a 4TB external drive to use with Time Machine. “This is going to fill up my external drive fairly quickly,” he worries.

Time Machine just looks like it’s making a full backup every time it creates one, but it’s really using a clever technique to only copy changed files at each hourly interval.

While Time Machine’s method has been the same since its introduction, it’s opaque to most users, because it’s meant to work without requiring any maintenance or fussing. (In fact, that’s a flaw, too, because there are no tools to fix corruption or control any of the settings, like frequency and purging of older files.)

Time Machine only copies a file when it changes, but it creates snapshots for each backup that use what are known as hard links for every file on your backed-up drive. A hard link looks and acts like a separate copy of a file when you examine it in the Finder or via the Terminal. So if you have a file called House Survey 541B.docx in every snapshot in Time Machine, you’d be forgiven for thinking that it was duplicated all those times and Time Machine had made a few dozen copies of House Survey 541B.docx.

However, the hard link means that the file exists uniquely just once on a drive. Every instance of the file you see is really a link to that one unique version. Each of those links can be removed without deleting the original until there is just a single link left. When that’s removed, the associated file is deleted too.

The advantage of this hard-link system is both user-friendly navigation in the Finder, and also a straightforward method of restoring a snapshot without having to perform additional date and time comparisons or other operations. It also means you can delete snapshots without having to worry about files being deleted that are required for other backups.

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